1. Field of the Invention
This invention generally relates to the field of photolithography as employed for the fabrication of micro- and nano-structures, and it relates more particularly to the field of Talbot imaging as employed for transferring periodic patterns of features defined in a mask onto a photosensitive layer on a substrate.
2. Description of Related Art
Lithographic fabrication enables the creation of micro- and nano-patterns on surfaces. Photolithographic techniques involve exposure of a photosensitive surface to a light-field with a certain pattern of intensity distribution. The surface usually consists of a thin layer of sensitive film, such as a photoresist, coated onto a substrate surface. Chemical or physical changes that occur in the photoresist may be used in subsequent processes to obtain desired patterns of materials on the substrate surface. In the most commonly used photolithographic technique the pattern is defined in a mask and the pattern is transferred to the substrate by illuminating the mask and imaging the light transmitted by the pattern onto the substrate surface using an optical system.
For many applications patterns are required that comprise a unit cell of pattern features that repeat in one or two dimensions, that is, periodic patterns. A specialized photolithographic technique based on the Talbot effect is advantageous for transferring such periodic patterns from masks onto substrates because it avoids the use of an imaging system which, for high resolution patterns, is complex and high-cost. With this technique a mask defining a periodic pattern is illuminated with collimated beam of monochromatic light and the light diffracted by the pattern reconstructs images of the periodic pattern at certain distances, called Talbot planes, from the mask (see, for example, C. Zanke, et al., “Large area patterning for photonic crystals via coherent diffraction lithography”, J. Vac. Sci. Technol. B 22, 3352 (2004)). In the case of a one-dimensional periodic patterns, in which a unit cell of features repeats in one direction, the separation, s, between successive Talbot planes is related to the illumination wavelength, λ, and period of the pattern, p, bys≈kp2/λ  equ. (1)where k is a constant whose value depends on the array-type of the periodic pattern.
For a one-dimensional periodic pattern, i.e. a linear grating, k=2, whereas for two-dimensional periodic patterns, the value of k depends on the array symmetry. For a hexagonal close packed pattern of features, k=1.5 if p refers to the nearest-neighbour distance.
The Talbot effect, which is also called self-imaging, may be used to print periodic patterns onto substrates. Midway between the Talbot planes there are other images with the same period that are phase shifted by half the period with respect to those in the Talbot planes. Furthermore, between these phase-shifted images and the self-images there are so-called Talbot sub-images that have smaller periods, i.e. higher spatial frequencies. By placing a photoresist coated substrate at one of these fractional Talbot planes, periodic patterns can be printed with a spatial frequency that is a multiple of that in the original mask. This variant, which enables spatial-frequency multiplication, performs better when the duty cycle of the periodic pattern in the mask is optimized to yield a high-contrast intensity distribution in the fractional Talbot planes (see U.S. Pat. No. 4,360,586). In the prior art, it is also known that by fabricating the periodic patterns in the mask out of phase shifting materials the contrast of the Talbot images can be enhanced. With the Talbot technique, however, the intensity distribution of the imaged pattern is very sensitive to the distance from the mask. Therefore, precise positioning and alignment of the substrate to be patterned with respect to the mask is of critical importance. This “depth of field” (DoF) consideration becomes much more restrictive as the period of the pattern in the mask is reduced because the DoF depends on the square of the pattern period. This limitation is especially severe if the periodic patterns need to be printed onto substrates that have imperfect flatness or have topographical features on their surface.
In another, related method known in the prior art as “near field holography”, a mask with a periodic pattern is illuminated obliquely by a collimated beam of monochromatic light (see, for example, D. M. Tennant, et al., “Characterization of near-field holography grating masks for opto-electronics fabricated by electron beam lithography, J. Vac. Sci. Technol. B 10, 2530 (1992)). The angle of incidence of the illuminating beam is chosen in relation to the period of the pattern and to the wavelength of the light so that only the zeroth-order (i.e. undiffracted) beam and a first-order diffracted beam are present in the transmitted light-field. An image of the periodic pattern is formed by the interference of these two transmitted beams. A major drawback of this method is that the pattern transferred to the substrate necessarily has the same period as the pattern in the mask, in other words there can be no gain in resolution with respect to the mask. Furthermore, only one-dimensional, line/space periodic patterns can be transferred with this method, which is a considerable disadvantage. Additionally, in order that the imaged pattern has a good contrast this method requires the use of expensive phase masks that are usually formed by etching the patterns into fused silica substrates. Moreover, this method requires obliquely incident light which results in a rather cumbersome system configuration, and requires polarized light which can further complicate the illumination system and reduce the utilization efficiency of the light source.
Achromatic Talbot lithography has recently been introduced as a new method for printing high-resolution periodic patterns in a cost effective way (see H. H. Solak, et al., “Achromatic Spatial Frequency Multiplication: A Method for Production of Nanometer-Scale Periodic Structures”, J. Vac. Sci. Technol., 23, 2705 (2005) and European Patent Application No. 05803386.1). It offers two significant advantages for lithographic applications: firstly, it overcomes the depth-of-field problem encountered in the classical Talbot method described above, and secondly, for many pattern types the printed patterns have a higher spatial-frequency than that in the mask, that is, it can perform a spatial-frequency multiplication. Achromatic Talbot lithography (ATL) illuminates the mask with a collimated beam from a broadband source and the substrate to be patterned is placed at or beyond a certain distance from the mask at which the image generated becomes stationary, that is, invariant to further increase in distance. The minimum distance, dmin, required for the stationary image to be formed is related to the period of the pattern, p, in the mask and to the spectral bandwidth of the illumination, Δλ, by:dmin≈kp2/Δλ  equ. (2)                where k is a constant whose value depends, as for equ. (1), on the array-type of the periodic pattern.        
At this distance the Talbot planes for the different wavelengths are distributed in a continuous manner, and so placing the substrate at or beyond this distance exposes the substrate to the entire range of lateral intensity distributions that occur between successive Talbot planes for the individual exposure wavelengths. The pattern printed onto the substrate is therefore the integration, or average, of this range of distributions, and this is insensitive to increasing distance between the substrate and mask. This property of a stationary image is also entirely different to the behavior of the images of mask patterns produced by conventional projection, proximity or contact lithography techniques, for all of which the images show a strong variation in the direction of image propagation, thus restricting their range of application.
If ATL is applied to one-dimensional patterns of the line/space type, the stationary image printed onto the substrate usually exhibits spatial-frequency multiplication: the period of the pattern is generally reduced by a factor of two. In the case of two-dimensional patterns, the spatial-frequency of the printed pattern depends on the arrangement of the features in the mask. For example, if the mask has an array of clear holes on a square grid the ATL image generally consists of intensity peaks on a square grid with a period that is smaller than that in the mask by a factor of √2. On the other hand, when the mask has an array of holes on a hexagonal grid the ATL image generally consists of intensity peaks on a hexagonal grid with the same period. The intensity distribution in the ATL image produced by a particular mask pattern may be determined using modeling software that simulates the propagation of electromagnetic waves through masks and through space. Such simulation tools may therefore be used to optimize the design of the pattern in the mask for obtaining a particular printed pattern at the substrate surface.
The ATL method has been developed primarily to print periodic patterns that comprise a unit cell that repeats with a constant period in at least one direction. The technique may, however, also be successfully applied to patterns whose period spatially varies in a sufficiently “slow”, gradual way across the mask such that the diffraction orders that form a particular part of the stationary image are generated by a part of the mask in which the period is substantially constant. The tolerance to such variation in period may be determined using modelling software of the kind mentioned above, and the patterns concerned may be characterized as being quasi-periodic.
A drawback of ATL is that it requires a relatively large separation between the mask and substrate, and so the spatial coherence and collimation of the illumination beam need to be much higher. For certain light sources, such as arc lamps, this is a problem because higher spatial coherence and collimation can only be achieved by greater spatial filtering, and this reduces the power in the illumination beam, which is undesirable for a production process.
Increasing the separation between mask and substrate also degrades the edges of the printed pattern. This occurs, firstly, because of Fresnel diffraction effects at the edges of the light-field transmitted by the mask, which get stronger as the propagation distance increases; and, secondly, because the different diffracted orders in the transmitted light-field diverge as they propagate, and so at the edges of the light-field there is imperfect overlap between the orders and therefore imperfect image generation, which gets worse with increasing separation.
The advantages offered by the ATL technique may also be obtained using another prior-art modification of the classical Talbot method. In this alternative scheme, which is disclosed in U.S. Pat. Appl. no. 2008/0186579, the periodic pattern in the mask is illuminated by a collimated beam of substantially monochromatic light and during exposure the substrate is displaced longitudinally relative to the mask by a distance corresponding substantially to an integer multiple of the separation between successive Talbot image planes. The technique, which may be called Displacement Talbot lithography (DTL), also results in the substrate being exposed to the entire range of lateral intensity distributions that occur between successive Talbot image planes, thereby also producing an integration, or averaging, of the range of lateral intensity distributions over the course of the exposure. Whereas the effects of the ATL and DTL techniques are essentially the same, in that they both produce stationary images with a large depth of focus and enable spatial-frequency multiplication, the DTL scheme also operates well with much smaller separations of the substrate and mask, so is advantageous in view of degradation at pattern edges and utilization efficiency of the light source. Further, the DTL technique is more suitable for printing high-resolution patterns over high topographies on the substrates; and it allows the use of laser source, which can be desirable for a production process.
A enhancement of the DTL technique is described in un-published U.S. application Ser. No. 13/035,012 which is incorporated herein by reference. This application teaches that the exposure dose per incremental displacement of the photoresist-coated substrate relative to the mask be varied during the displacement instead of being constant. Preferably, the exposure dose per incremental displacement of the photoresist-coated substrate relative to the mask is varied according to a substantially Gaussian or similar profile whose FWHM corresponds to at least the Talbot distance of the light-field transmitted by the periodic pattern in the mask. It further teaches that the variation may be obtained by varying at least one of the speed of displacement and the intensity of the exposure beam during the displacement of the substrate with respect to the mask. This enhancement of the DTL technique enables periodic or quasi-periodic patterns to be printed more uniformly and reproducibly onto substrates than the original DTL method for which the size of the printed features is determined to be somewhat sensitive to deviations of the longitudinal displacement of the substrate from the specific values (integer multiple of the Talbot distance. This variant of the DTL technique further enables periodic patterns to be printed uniformly and reproducibly when the presence of 2nd or higher diffraction orders in the transmitted light-field from the mask prevents an exact Talbot imaging and an exact Talbot distance; and additionally enables two-dimensional periodic patterns of features to be printed uniformly and reproducibly printed onto substrates when the periods of the pattern are different along different axes or when the pattern also generates diagonally diffracted orders. It moreover enables patterns of features to be printed uniformly and reproducibly onto substrates when the period of the pattern in the mask is not constant but varies either continuously, as in chirped gratings, or step-wise across the mask. The form of the patterns printed by DTL at the substrate surface using one-dimensional and two-dimensional patterns in the mask are essentially the same as for ATL and can be determined using similar modelling software as mentioned above.
As for ATL, DTL is not restricted to purely periodic patterns but can also be applied to quasi-periodic patterns.
In fact, the same averaging of the entire range of lateral intensity distributions between successive Talbot image planes may also be achieved using a combination of the ATL and DTL methodologies. For example, if a (non-monochromatic) source with a bandwidth, Δλ, is employed and the substrate is arranged at a distance d<dmin (see equ. (2) for ATL), then the required averaging effect may still be achieved by additionally longitudinally displacing the substrate relative to the mask, the displacement needed being less than that required by just DTL. By arranging d<dmin, the undesirable consequences of a large separation between substrate and mask can be reduced.
Using the ATL or DTL techniques (or a combination of partial forms thereof), it can be difficult and/or expensive, however, to design and realize mask patterns that can generate the intensity distributions required at the substrate surface for certain applications. Furthermore, they are too restrictive for generating images with intensity profiles that have large enough gradients at the edges of the features and have sufficiently high contrast for ensuring a high-yield production process. It is, moreover, desirable for some applications that the shapes of the holes printed from a particular mask can be changed without changing the mask. It is furthermore especially desirable that the period of the pattern printed from a particular mask can be significantly smaller than that e achieved using the prior-art teaching standard ATL or DTL. A smaller period of printed pattern can be advantageous for obtaining higher performance from the manufactured device, as well as enabling the printed patterns to be employed in other fields of application. Alternatively, enabling a larger period of pattern in the mask for a particular period of printed pattern is advantageous for reducing the manufacturing cost of the mask, especially if the size of the pattern is large.
It is therefore the purpose of the present invention to provide a solution to overcome the above-described limitations and disadvantages of achromatic Talbot lithography and displacement Talbot lithography as taught in the prior art. Specifically, it is a first object of the invention to provide a cost-effective and more versatile lithographic method and apparatus for printing a larger variety of high-resolution patterns onto substrate surfaces to satisfy a range of fields of application and for ensuring a high-yield production process. It is a second object of the invention to provide a lithographic method and apparatus that enable a more efficient utilization of light from the illumination source in order to improve productivity and reduce costs in the manufacturing process. It is a third objective of the present invention to provide a lithographic method and apparatus that enable the period of the pattern printed from a particular mask to be significantly smaller, and the resolution of the printed features to be substantially higher, than the respective values achieved using the prior-art teaching ATL or DTL.